1. Field of the Invention
The present invention relates to a valve for gradually supplying a fluid under pressure to an actuator to slowly start the actuator, and more particularly to a slow starting valve for holding a fluid under pressure for a predetermined period of time with a needle valve when a valve body is opened by a solenoid-operated valve, to start an actuator at low speed.
2. Field of the Invention
There have heretofore been widely used actuators which are reciprocally operated when supplied with a fluid under pressure upon opening and closing operation of a solenoid-operated valve. If the power supply for the solenoid-operated valve fails while the actuator is in operation or the power supply is restored after the actuator is shut off in case of emergency, then the actuator may not be controlled properly and may operate in error because of a fluid under pressure that remains in the actuator, a fluid supply passage, or a fluid discharge passage.
To solve the above problems, there has been employed a slow starting valve 2 as shown in FIG. 1 of the accompanying drawings. The slow starting valve 2 comprises a block 10 having an inlet port 4, an outlet port 6, and a discharge port 8 which are defined therein, a first valve body 14 disposed in a passage 12 which provides communication between the inlet port 4 and the outlet port 6, and a second valve body 16 disposed in the body 10.
The first valve body 14 has a smaller-diameter lower valve member 18 for closing the passage 12. The second valve body 16 has a valve member 20 for preventing communication between a chamber 32 in the block 10 and the discharge port 8. The valve members 18, 20 are normally urged to move upwardly in FIG. 1 by respective springs 22, 24. The valve members 18, 20 are normally seated on respective valve seats 26, 28.
A needle valve 30 is disposed in the block 10 below the inlet port 4. The needle valve 30 has a pointed distal end facing a passage 34 communicating with the chamber 32 that is defined between the first and second valve bodies 14, 16. A passage 36 is defined in the block 10 and extends from the inlet port 4 across a manually movable spool valve 38 to a valve body 42 of a solenoid-operated valve 40.
The spool valve 38 is housed in a chamber 44 from which a passage 46 extends to a valve member 48 of the second valve body 16. A passage 50 which also extends from the valve 44 goes to the valve body 42 of the solenoid-operated valve 40. The first valve body 14 also has a larger-diameter valve member 52 housed in a chamber that is vented to the atmosphere through a passage 54. The valve member 52 has a pressure-bearing surface communicating with the outlet port 6 through a passage 56.
When the solenoid-operated valve 40 is inactivated and the spool valve 38 is in the position shown in FIG. 1, a fluid under pressure introduced from the inlet port 4 flows through the passage 34 across the needle valve 30 into the chamber 32. Since the valve members 18, 20 in the chamber 32 are seated on the respective valve seats 26, 28, the fluid pressure does not flow out of the outlet port 6.
When the solenoid-operated valve 40 is actuated, the valve body 42 is opened, allowing the fluid under pressure introduced from the inlet port 4 to flow through the passage 36 across the spool valve 38 into the passage 50, and then from the chamber 44 through the passage 46, thereby displacing the valve member 48 of the second valve body 16 downwardly in FIG. 1. As a result, the valve member 20 of the second valve body 16 is unseated from the valve seat 28, so that the fluid under pressure introduced from the inlet port 4 flows through the passage 34 and the chamber 32 into the outlet port 6, from which the fluid under pressure is gradually supplied to an actuator (not shown) connected to the outlet port 6.
The pressure of the fluid that flows into the outlet port 6 is applied through the passage 56 to the valve member 52 of the first valve body 14. In the case where the coil spring 22 has a sufficiently large resilient force, the fluid pressure supplied via the needle valve 30 and applied to the valve member 18 and the resilient force of the coil spring 22 are large enough to overcome the pressure from the larger-diameter valve member 52, and hence the first valve body 14 is not opened. Since the amount of the fluid flowing through the needle valve 30 to the outlet port 6 is very small, the amount of the fluid supplied to the actuator is also very small, making it possible to start the actuator gradually, i.e., slowly.
When the fluid pressure applied to the valve member 52 is gradually increased, the first valve body 14 is lowered in FIG. 1 while overcoming the fluid pressure acting on the valve member 18 and the resilient force of the coil spring 22. The fluid under pressure introduced from the inlet port 4 is therefore supplied directly to the outlet port 6.
When the fluid under pressure flows after the first valve body 14 is displaced downwardly, an appreciable pressure drop is developed, lowering the fluid pressure in the outlet port 6. As a result, the fluid pressure in the passage 56 drops, and the first valve body 14 is displaced upwardly under the fluid pressure acting on the valve member 18 and the resilient force of the coil spring 22, blocking the fluid pressure from the inlet port 4. Consequently, the first valve body 14 is closed after only a large amount of fluid under pressure is consumed. The actuator still operates slowly, and the pressure drop may cause erroneous operation of the pilot-operated valve.
The above drawback may be eliminated by increasing the area of the pressure-bearing surface of the valve member 52 of the valve body 14. However, the increased area of the pressure-bearing surface of the valve member 52 may cause the valve body 14 to open too early, and the actuator which has been started slowly may tend to operate quickly before it reaches the stroke end.
With the above arrangement, unless the area of the pressure-bearing surface of the valve member 52 is considerably increased, the valve body 14 cannot be displaced downwardly against the valve member 18 which is urged upwardly by the fluid pressure introduced from the inlet port 4 and the resilient force of the coil spring 22 because the primary fluid pressure from the inlet port 4 is applied to the lower surface of the larger-diameter valve member 52 and the fluid pressure lower than the primary fluid pressure is applied to the upper pressure-bearing surface of the valve member 52. Accordingly, the conventional slow starting valve shown in FIG. 1 is necessarily large in size.